Moreno, who accounted for 1,586 total yards and 13 touchdowns in his fifth NFL season, is far more accomplished as a NFL tailback than Miller. But Miller has no interest in stepping aside, relinquishing the starting spot he held last season with the Miami Dolphins.

That explains why Miller has spent the past three weeks as the Dolphins starting running back. During day one of the team's three-day minicamp Moreno sparingly worked, and when he did get reps during team drills he was working behind Miller and Daniel Thomas.

“I’ve been competing my whole life. Whether it was at the University of Miami or in the NFL,” said Miller, who accounted for 879 total yards and two touchdowns last season. “[Competition] bring the best out of all players….I want to get better as a player too, so I’m up to the competition.”

Miller had a couple of impressive runs during the Dolphins' team period on Monday. One of them would have been a 40-yard sprint for a touchdown out of a draw. Miller was so quick getting away from the trenches he wasn't touched by any of the second-team defenders.

Head coachJoe Philbin stiff armed questions about Moreno, who signed a one-year, $3 million deal as a free agent, saying the coaching staff has been pleased with what they've seen from the former Broncos starter.

However, Philbin wouldn't explain why the former University of Georgia standout has been working with the third team offense for the past few practices. He simply pointed out Miller and Thomas are tailbacks Miami's giving the reps to.

"Knowshon's had a good camp. He brings a lot of energy into the building every single day. He's very good in the meeting rooms," Philbin said of Moreno, whom he previously admitted could be in better shape physically. "He's picked up the system well and he's competed hard on the practice field."

The Dolphins practice again Wednesday morning and Philbin said the team will close out minicamp with a spring scrimmage, where he'll split the team in half.

Each tailbacks performance the next two days might dictate where they start training camp on the depth chart.